1. Field of the Invention
The present invention relates to a recording apparatus based on electrophotographic method for a printer, copying machine, facsimile, or the like, and, more particularly, to a fixing device and fixing method excellent for fixing a toner image onto a recording medium, and a recording apparatus.
2. Description of the Related Art
A conventional recording apparatus based on electrophotographic method is involved in a process for making visible a toner image on a recording medium, and in a process for fixing the resulting toner image onto the recording medium. The heated toner image is fused onto the recording medium, and then cooled and solidified. In the fixing process, utilizing the properties of the toner, it is heated and fused, and then naturally cooled so that it is fix onto the recording medium.
In a conventional fixing device, in the fixing process, a pair of fixing rollers, made up of one roller and one supporting roller, is press-contacted. At least one of the fixing rollers is heated to bring the pair of fixing rollers into contact with each other, thereby forming a nipping and fusing section. The recording medium is passed through this nipping and fusing section to fix the toner aligned thereon. When the recording medium is passed through the nipping and fusing section, the aligned toner which forms an image on the recording medium is heated and at the same time subjected to pressure. The heat energy and pressure applied at the nipping and fusing section changes the shape of the toner. This action causes the toner to be fixed onto the recording medium. Here, only one of the fixing rollers needs to be heated. The toner image formed on the recording medium passes through the nipping and fusing section so that its toner-image bearing surface and the heated fixing roller contact each other. The heated heat fixing roller is called the heat roller, while the supporting roller is called the back-up roller. The heat roller and the back-up roller together may be called a pair of fixing rollers. In addition, either of the heat roller or the back-up roller may be called a fixing roller.
The heat roller is a hollow cylinder made of aluminum and has a heater at its central section. The aluminum hollow cylinder is called a core metal. A halogen lamp is often used for the heater. When this fixing device is used to heat the toner image, offsetting may occur in which the fused toner gets stuck onto the heat roller. When fixing is carried out for the second time, offsetting causes the toner to be transferred from the heat roller onto the recording medium, which results in misprinting. To prevent this, a material having high releasing capability such as silicone, rubber, fluorocarbon rubber, or fluorocarbon resin is used to form a parting layer, which is the surface layer of the heat roller which contacts the toner. In general, fluorine, silicone rubber, or fluorocarbon rubber is used for the surface layer. In particular, fluorocarbon resin is widely known for its high releasing capability. Examples of fluorocarbon resin surface layer materials which are frequently used include perfluoroalkoxy copolymer (PFA) and polytetrafluoroethylene (PTFE). The parting layer, which has a lower thermal conductivity than the core metal, acts as a thermal resistance section during fixing.
On the other hand, the back-up roller or the supporting roller has an elastic layer formed along the outer periphery of its metallic rotating body. When it is press-contacted with the heat roller, the elastic layer changes shape to form a nipping and fusing section.
Fan fold paper is primarily used for the recording paper. In the longitudinal dimension, it has perforations spaced apart at certain intervals. This type of recording medium is also called continuous paper.
In a conventional method of fixing a toner image by transporting continuous paper at a high Speed, the pair of fixing rollers do not provide enough heat for the toner. Therefore, a preheater or other such means is placed at a location just before the recording medium passes through the pair of fixing rollers. This preheater is often a hot plate which is positioned in the transporting path. It performs preheating from the back surface of the toner-image bearing surface of the continuous paper, which makes it necessary to supply heat corresponding to the thickness of the recording medium. This is not thermally efficient. The preheater, itself, must be made larger for sufficient and effective preheating, with the result that more space must be provided for the preheater than for the pair of fixing rollers. Such a prior art described above is disclosed, for example, in Japanese Patent Laid-Open No. Sho 64-9483 and Japanese Patent Laid-Open No. Hei 25-504634.
Merely removing the preheater does not solve the above-described problem present in the conventional fixing device. This is because it is difficult to provide the necessary heat for fixing the toner image by only using a pair of fixing rollers alone. When the parting layer of the heat roller is reduced in thickness, its thermal resistance decreases. As the amount of heat applied to such a thin parting layer increases, the wearing resistance of the heat roller is considerably reduced. This in turn reduces the heat roller life, so that the fixing device cannot be used for practical purposes. In preheating the recording medium which is brought into contact with the heater before it reaches the nipping and fusing section or forwardly of the heat roller, the toner is only accumulated on the recording medium and is not adhered thereon. In addition, the recording medium, which is not nipped by the back-up roller, undergoes thermal contraction. This causes a slippage between the recording medium and the heat roller surface, giving rise to image slippage.
To solve the above-described problems, a method may be used to allow the recording medium to remain in contact with the heat roller even after it has passed through the nipping and fusing section. Since the toner image is heated while it is nipped between the rollers, it is adhered onto the recording medium without the occurrence of image slippage. The condition of the toner image immediately after it has been nipped is not satisfactory from the viewpoint of fixing because there is not enough adhesion force. However, even if, a slippage occurs between the recording medium and the heat roller surface due to thermal contraction of the medium, image slippage will not occur since there is some adhesion force between the toner and the recording medium. Since thermal contraction of the recording medium is considerable immediately after heating, a substantial amount of thermal contraction force is developed in the nipping and fusing section. However, the medium is nipped by the back-up roller, so that thermal contraction, itself, does not occur. Thermal contraction of the recording medium is slight in the contact region after it has passed through the nipping and fusing section. When this method is taken, the following technical problems need to be overcome.
(1) For a thick recording medium, the same conventional method is used to increase the amount of heat provided by the heat roller, which is required for overcoming fixing failure. When the thick medium is subjected to the same heating conditions as a thin one, the toner is fixed less strongly onto the recording medium. Therefore, one technical problem which needs to be solved is how to increase the fixing strength of the toner onto the recording medium irrespective of the medium thickness.
(2) The same conventional method is used to provide more heat to the central portion of the heat roller as compared to the end portions of the heat roller extending beyond the width span of the: recording medium. The difference between the amount of heat supplied to the central portion and that supplied to the ends gradually increases, so that when printing is performed successively for a long period of time, the temperature of the central portion of the heat roller drops significantly. This results in a marked temperature difference distribution in the longitudinal direction of the heat roller. For this reason, a fixing failure occurs, in particular, at the central portion of the heat roller, in which the toner is less strongly fixed onto the recording medium. Consequently, in carrying out printing successively for a long period of time, another technical problem is how to control temperature drop of the central portion of the heat roller to prevent fixing failure thereof.
(3) After thermal energy and pressure have been applied to the recording medium and toner at the nipping and fusing section, heat energy is supplied from the contact region thereto. For this reason, more work is done by thermal energy required for fixing by proportion than by pressure. As a result, after fixing, more time is required for the temperature drop of the recording medium, which slows down the solidification of the toner. Because of this, the toner remains fused even in a paper stacker which stacks recorded paper, so that when the recording medium is folded a phenomenon called toner stick occurs in which the toner adheres onto other portions of the recording medium surface not requiring fixing, and other toner images. When toner stick occurs, the necessary information is transferred onto other recording medium surfaces, so that after recording unnecessary information may be recorded or necessary information may be missing to give rise to image loss. Another technical problem is how to prevent misprinting and image loss due to toner stick.